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You hit the primary need for days off - the mental reset - out of the park.
 
But, IMO, you whiffed on the other variable -- the ability of the body to repair the muscular damage done by playing athletics at the highest level.  For mortal man, science says it actually takes 36-48 hours to repair damage from strenuous physical activity.  (Body builders train every other day because the rest day allows faster increase in muscle mass).
 
I believe one of the things that sets pro athletes apart is that they repair faster - under 24 hours.  So, they CAN play every day without detrimental effect.  (The steroid era may be the ultimate proof of the reality in baseball - as several players admitted to dipping into the pharmacy in response to injuries).
 
The thing about aging is this ... the speed at which the body repairs the micro-muscle tears slows down.  The Beltre disaster season, (I believe) was an absolute cause and effect of him being "a little" injured and never getting enough time to fully heal - (until he finally went on the DL months too late).
When you're trying to make precise contact with a spheroid hurtling at 90 mph, and shifting in the other 2 dimensions at the same time, "tiny" disruptions in physical well being can have all kinds of impacts on results.  While the typical pro-athlete mantra is "suck it up" and "play through the pain", there is a point where any athlete is "just" uncomfortable enough that they can play - but cannot produce.
 
I believe, the 37 year old Ichiro has likely moved beyond the point where his body can fully repair his daily muscle damage.  So, what happens is it starts accumulating.  For a week or two, no detrimental effect.  But, after enough consecutive days played, maybe a tricep is a little sore, and his ablity to change the vertical plane of the bat is ever-so-slightly compromised. 
A "three day weekend" out of the lineup, and Ichiro might recover and find his swing again.  BUT ... if you then return him to his Ripken status and ride him, he'll likely as not go cold again.
 
The mental reset is certainly the larger variable for any younger player.  But for guys above age 35 ... I would argue that playing 5 days a week instead of 6 should be a nearly mandatory adjustment.  It's not.  But considering how well we saw some codgers play in their late 30s while on steroids ... I'd love to see some managers 'baby' their aging Hall of Famers more often ... because I think it would extend the careers of many of them.